1. Field of the Invention:
This invention relates to improvements in apparatus for controlling the relevelling of an elevator car.
2. Description of the Prior Art:
An elevator car is normally suspended from a main rope, or cable, and moves vertically in a hoistway. When the vertical travel of the car is considerable, the amount of stretching of the main rope varies considerably, according to the load in the car. This is particularly obvious in the case of high rise and super high rise buildings, where the rope is necessarily long and the load applied to it varies considerably with the loading and unloading of passengers. Accordingly, as the load in the car varies after the car has arrived at a floor and stopped, and the main rope is maintained by a brake, the car may move up or down, such that any step between the car floor and the building floor increases to the point that it may eventually go beyond the permitted landing error (hereinbelow referred to as "landing error"). In such circumstances, in order to place the car back within the landing error, the brake is released, and the car is moved in a direction opposite to the aforementioned movement up or down. This is called "relevelling".
As is generally known, relevelling is carried out with both the car and the landing doors open, so the domain in which relevelling is possible is, from the point of view of safety, necessarily narrow. Also, if the attempt at relevelling fails, and the car moves beyond a predetermined distance from the landing floor, the car must immediately be restrained.
Immediately after the brake has been released for the purpose of relevelling, there is a tendency for the extent of movement (extent of misalignment with the landing floor) of the car to increase due to the weight difference between the car and its counterweight. In an elevator which has a torque balancing means, whereby the load in the car is detected prior to starting up the car, and an armature current is caused to flow which offsets the unbalancing torque which arises from the weight difference between the car and the counterweight, the extent of movement is comparatively small. However, in an elevator without a torque balancing means, the extent of movement is quite large, and particularly when the car is overloaded, this can be marked to the extent that the car may fall below the domain in which relevelling is possible. Also, if for some reason the brake is unable to produce a sufficient arresting force to support the overload, once the car has gone beyond the domain in which relevelling is possible, the means for maintaining the car are removed, and the car may slide downwards, which is very dangerous.